1. Field of the Invention
The present invention relates to a technique preferably applied to an acoustic measurement method.
More particularly, the present invention relates to a technique for non-intrusively measuring noise caused by a jet flow.
2. Description of the Related Art
In the development of the next generation's supersonic transport, reduction of the noise caused by the exhaust of a jet engine (also called “jet noise”) is one of the most important problems. The jet flow of the jet engine is sheared off from the ambient atmosphere, so that a strong turbulence is generated in a boundary region (called a “shear layer” or “mixing layer”) between the jet and the ambient atmosphere. In the case where the jet flow is a supersonic jet, for example, a screech (The word “screech” means a sharp piercing cry, also means a very intensive noise having a particular frequency. In the aviation industry and the like, the aforesaid jet noise is referred to as a “jet screech” or simply a “screech”) will be caused due to the turbulence generated in the boundary region. Further, along with the screech, a broadband noise appears which includes sounds at various frequencies. Since the screech is caused because a large amount of energy is concentrated in a very narrow frequency band, the screech becomes a serious problem which not only leads to noise pollution, but also leads to damage of the framework of the vehicle (see Raman, G., J. Sound and Vibration, 225-3 (1999), 543-571).
Various efforts are being made in order to reduce the noise such as the jet screech and the broadband noise. These efforts include: conducting a jet flow analysis (see Powell, A. et al., J. Acoust. Soc. Am., 92-5 (1992), 2823-2836; and Umeda, Y. and Ishii, R., Int. J. Aeroacoustics, 1-4 (2002), 355-384), developing a method for reducing the screech by providing projections (also called “tabs”) on the nozzle exit of the jet engine (see Ahuja, K. K. and Brown, W. H., AIAA paper 89-0994; Samimy, M. et al., AIAA Journal, 31-4 (1993), 609-619; and Kobayashi, H. et al., ASME NCA, Acoustic Radiation and Wave Propagation, 17 (1994), 149-163), developing a method for reducing the screech by providing aerodynamic tabs which inject small gas jets from the nozzle wall, instead of mechanical tabs (see Araki, M. et al., JSME Journal (Series B), 71-707, B (2005), 1798-1805; Araki, M. et al., AIAA Journal, 44-2 (2006), pp. 408-411; and Araki, M. et al., JSME Journal (Series B), 73-726, B (2007), 567-574).